Holey metal, Batman! Extraordinary optics make it appear transparent

Two wave types contribute to extraordinary optical transmission through metal.

Take a very thin sheet of metal and drill it with tiny holes in a regular rectangular pattern. Ordinarily, if you shine light with wavelength that is larger than the holes, it wouldn't get through—the metal would be opaque. However, in the case of this particular pattern of holes, a lot of the light gets through the sheet anyway, a phenomenon known as extraordinary optical tranmission (EOT). Since the discovery of EOT, the effect has been harnessed in a number of optical and biophysical devices. A full theoretical understanding of the phenomenon proved elusive, however, which could hamper further device development.

A systematic exploration of hole spacing may help elucidate the mechanism behind EOT. Frerik van Beijnum and colleagues demonstrated that electrons on the metal's surface have two properties that contribute to EOT, with different strengths depending on the hole density and configuration. These results enabled the researchers to determine the physical parameters that dictate EOT, potentially leading to new device designs.

Ordinarily, light can pass through an opaque barrier only if the barrier is pierced with openings larger than the light's wavelength. (This also applies to all manner of waves, including sound and water waves.) That's why EOT is fascinating: the holes are smaller than the wavelength, yet a substantial amount of light still gets through something that would ordinarily be opaque. Oddly, making the material thinner—and therefore more transparent—decreases the EOT effect.

Surface plasmon polaritons

A plasmon is a quasiparticle, a particle-like entity, created by the collective activity of electrons inside a plasma or metal. (Plasmas are materials, usually gases, in which electrons separate from their atoms, creating an electrically neutral substance. Metals are effectively solid plasmas.) Polaritons are another type of particle-like excitation, this time arising from the interaction between electrons and light. Thus, a surface plasmon polariton is a quasiparticle generated when light strikes a metal surface; it acts much like a particle, being indivisible and having specific properties like electric charge, spin, and so forth.

The type of material makes a difference, however. So far, EOT is a phenomenon confined to metals, materials in which the outer layer of electrons in the atoms forms a more-or-less free-flowing fluid. That property explains metals' excellent electrical conductivity. Electron mobility also contributes to surface plasmon polaritons, in which the light directly excites the electrons into motion, creating waves that cross the surface of the metal. These waves are what transfer the light involved in EOT across the material—the light itself never passes through the holes.

However, prior experiments showed that surface plasmon polaritons by themselves cannot explain EOT. Researchers proposed quasi-cylindrical waves (QCW) as a secondary mechanism; these are waves in the electrons excited by the light striking the metal, centered at the holes. These waves also occur in non-metals, so they cannot be the sole driver of EOT either—but in combination with surface plasmon polaritons, they could successfully describe the entire anomalous transmission.

To test the effect of positioning of holes on EOT, the researchers created seven configurations laid out in a rectangular pattern. Each configuration kept the vertical spacing of the holes fixed but gradually increased the separation between the holes in the horizonal direction. To make the holes, the researchers started with a piece of glass with regularly spaced protrusions and overlaid it with thin gold and chrome foils. When they etched the protrusions in the glass away, it left evenly distributed perforations in the metal layers. For each of these configurations, they used the same wavelength of infrared light, known to produce EOT in these hole sizes.

As the new study showed, the positioning of the holes is essential for both the polariton waves and the quasi-cylindrical waves. The holes seem to provide a kind of artificial crystal lattice for the plasmons, and the farther apart the holes are placed, the smaller the quasi-cylindrical wave effect got. This is because cylindrical waves decrease in amplitude over distance, much as ripples in a pond get smaller the farther from the place where the rock fell in. They determined that, for widely spaced holes, EOT was dominated by surface plasmon polaritons, but for the more closely packed configuration, the QCW effects became significant.

Because they kept the vertical spacing of the holes constant, the researchers were able to separate the contributions from both factors in a clear way—a significant measurement in its own right. Understanding the phenomena driving EOT could help design of future devices and refinement of existing technology, which can have applications in diode lasers and photovoltaic solar cells.

...except that's not aluminum, but alumina. Which, in the form of various precious gemstones, has been available in several transparent varieties for a few million years longer than any Star Trek references.

The photons bouncing off the front of the mirror have to interact in some fashion with the electrons at the back of the mirror to display the pattern on the back. I have one of these mirrors and the bit about reflecting sunlight is key - reflecting a lightbulb indoors doesn't display the effect so luminosity plays a role in the mirror.

...except that's not aluminum, but alumina. Which, in the form of various precious gemstones, has been available in several transparent varieties for a few million years longer than any Star Trek references.

I was going to say the same thing. If they have special technology, it's in the fabrication, not the specific material itself.

...except that's not aluminum, but alumina. Which, in the form of various precious gemstones, has been available in several transparent varieties for a few million years longer than any Star Trek references.

I was going to say the same thing. If they have special technology, it's in the fabrication, not the specific material itself.

Well, sapphire is monocrystalline alumina, which is usually transparent. Transparent polycrystalline alumina is still hard to achieve in practice, because of scattering from the crystal grain structure.

When I got to take a tour of SpaceX they had a demo piece from a metal printing machine that was made of slightly transparent metal (there had to be a bright light source behind the object) or a fairly reasonable thickness to be a structural part... maybe 1/4" thick. I'm not sure if it was the same effect as what they're talking about in the article, but the any time you can look through a piece of metal is friggin' cool.

I am very disappointed that the article does not delve into details about the potential practical applications of this research.

Well, it could basicly be used anywhere where one needs a conductor that needs to be transparent, displays and solar panels for example. Indium oxide is the current material in use but indium is becoming more expensive. If for example aluminium could be made transparent enough it could be used instead of indium.

is this at all like the power/sleep indicator light on the old unibody macbooks (there's a bright white light indicator, but when it's off the metal just looks like ... metal--maybe there are holes but if so they are tiny).

is this at all like the power/sleep indicator light on the old unibody macbooks (there's a bright white light indicator, but when it's off the metal just looks like ... metal--maybe there are holes but if so they are tiny).

Pretty sure Apple just cuts really small holes and sticks an LED behind them. This is similar in that they use small holes, but in this case the holes are smaller than a wavelength of light. Therefore, light should not be able to pass through these holes, but does anyway.

Confusing stuff - should be interesting to follow! I also wish they had a photo of it functioning.

...except that's not aluminum, but alumina. Which, in the form of various precious gemstones, has been available in several transparent varieties for a few million years longer than any Star Trek references.

I was going to say the same thing. If they have special technology, it's in the fabrication, not the specific material itself.

Well, sapphire is monocrystalline alumina, which is usually transparent. Transparent polycrystalline alumina is still hard to achieve in practice, because of scattering from the crystal grain structure.

True, but there are ways around that if you're willing to consider composite materials.

I used to think Corningware was ceramic and their "visions" line was glass. Then one of their retired employees gave a seminar on the history and chemistry of glass, and it turns out both are glass-ceramic composites.

The classic Corningware is opaque because of light scattering from the interfaces between the crystalline and amorphous materials. The "Visions" line is transparent because the glass and ceramic have the same index of refraction, so there's no refraction or reflection at the interfaces.

Reading it a second time in a different way, cleared it up a bit for me.

The gist of it is that light transmission is not as simple as just being passed through, reflected, or absorbed, especially at the quantum level, there are more interactions can that affect its transmission. This has possibilities for new optical technologies that manipulate these effects at the quantum level.

Frankly I'm not surprised by this. After all quantum mechanics is still in infant form, and chemistry and materials science is still largely based in the classical physics realm, when really the basis for both lies in quantum mechanics.

Anybody else DESPERATELY disappointed at not seeing a photo illustrating EOT in actual occurrence through an appropriate sheet of metal?

Here is photo of the samples we used, illuminated from the back. The squares is where the holes are placed, and from left to right the number of holes is reduced. The gold is black, because no light is passing through it, nor is it illuminated with a sufficient amount of light to see the gold color. Each square is four tenths of a millimeter, so that is pretty small.